With respect to volume control of car audio, there is known a technique of controlling volume balance in accordance with the number of occupants in a vehicle and the positions of the occupants (for example, see JP-A-9-83277).
In the technique disclosed in the above publication, the number of occupants and the positions of the occupants are detected on the basis of image recognition and detection of body temperature, and the volume of sounds emitted from plural speakers (for example, every two speakers at each of front and rear seats, totally four speakers) is controlled in accordance with the detection results.
When the method disclosed in the above publication is implemented, an image pickup camera and a processor for executing analysis processing of images picked up by the image pickup camera are needed to detect the occupants on the basis of the image recognition. Furthermore, when the detection of occupants is carried out by the body temperature detection, it is required to provide a temperature sensor at each seat and a processor for executing occupant presence/absence judging processing based on the detected temperature. If such a complicated expensive device is adopted, the detection of occupants is surely performed. However, it would be more preferable to perform suitable volume control with a simpler construction. It is preferable that an acoustic effect which is a broader concept containing volume is set as a control target.